Which of the following is LEAST likely to cause a positive forced duction test?

• A. Thyroid eye disease
• B. Orbital tumor
• C. CN VI palsy
• D. Muscle entrapment following orbital fracture

Answer: (C)

Forced duction testing distinguishes mechanical restriction from neurogenic weakness by evaluating passive eye movement. If the eye cannot be moved in a certain direction despite gentle passive pressure, that indicates a mechanical restriction (scar, entrapment, mass, or tight muscle). If the eye can be moved passively without resistance, the problem is more likely neural or myopathic in nature.

Thyroid eye disease causes inflammatory swelling and eventual fibrosis of the extraocular muscles, producing a real mechanical limit to movement. An orbital tumor can physically compress or invade orbital structures, creating a mechanical block to movement. Muscle entrapment after an orbital fracture directly tethers a muscle within a fracture site, again a mechanical restriction. In all these, the forced duction test is positive.

CN VI palsy, however, is a neurogenic issue where the lateral rectus is weak but not physically blocked. The eye can be moved passively in the restricted direction, so the forced duction test is negative and least likely to be positive.